In recent years, attention has been drawn to OFDM (Orthogonal Frequency Division Multiplexing). The OFDM has such features that the receiver has a relatively simple circuit arrangement, the propagation path can be regarded as flat in terms of subcarriers, and it can easily be expanded into MIMO (Multiple Input Multiple Output).
According to the OFDM, since a plurality of carriers are multiplexed in time domain, the signal power exhibits a high peak when the carriers are brought into phase with each other in time domain. The high peak leads to an increase in PAPR (Peak to Average Power Ratio). As the increased PAPR reduces the coverage of the base station and increases the power consumption, there has been a need for reducing the PAPR.
Some PAPR reducing processes based on TR (Tone Reservation) have been proposed so far. For example, a TR with RT (Reserved Tones) process has been proposed in Multicarvier modulation with low PAR application to DSL and wireless, Jose Tellado, Kluwer academic Publications and OFDMA UL PAPR Reduction, R1-050891, Nortel, 3GPP TSG-RAN WG1 #42, London, UK, 29 Aug.-3 Sep. 2005. A TR without RT process has been proposed in PAPR reduction for OFDMA using improved tone reservation with low complexity, R1-050836, ZTE, CATT, 3GPP TSG-RAN WG1 #42, London, UK, 29 Aug.-3 Sep. 2005.
According to the TR with RT process, a subcarrier for a peak reducing signal is provided in addition to a subcarrier for a data signal, and a peak reducing signal for reducing peak power is generated in the subcarrier for a peak reducing signal. The peak reducing signal is added to the data signal, thereby reducing the peak power.
According to the TR without RT process, a peak reducing signal is generated in the same band as a data signal and with sufficiently low power compared with the data signal, and is added to the data signal, thereby reducing the peak power.
The TR with RT process has such features that it can reduce the PAPR without lowering the modulation accuracy (highering EVM) though the spectrum use efficiency is lowered as the reduction in the PAPR increases. The TR without RT process has such features that the spectrum use efficiency is not lowered though the modulation accuracy is lowered as the reduction in the PAPR increases.